The present invention relates to an apparatus for fabricating a substrate having a circuit pattern of, for example, a semiconductor device or a liquid crystal display. More particularly, the invention is concerned with a technique for inspecting a pattern on a substrate during manufacture.
Conventional electron beam type and optical type pattern inspecting apparatuses are disclosed in JP-A Nos. 258703/1993 and 160247/1999, respectively. More particularly, an example of an electron beam type inspection apparatus is disclosed in the JP-A No. 258703/1993, the construction of which is shown in FIG. 1.
As seen in FIG. 1, an electron beam 2 emitted from an electron beam source 1 is deflected in X the direction by a deflector 3 and is radiated onto an object substrate 5 through an objective lens 4. At the same time, while a Z stage 6 is moved continuously in the Y direction, secondary electrons, etc. from the object substrate 5 are detected by a detector 8, and the resulting detected signal is converted from analog to digital by an A/D converter 9 to provide a digital image. Then, in an image processing circuit 10, the digital image is compared with a digital image of a place on the substrate which is expected to be the same, and then a place giving rise to a difference is detected as a pattern defect, represented by pattern defect data 11, so that a defect position is determined.
An example of an optical type inspection apparatus is disclosed in the JP-A No. 160247/1999, the construction of which is shown in FIG. 2. In the this apparatus, light from a light source 21 is radiated onto an object substrate 5 through an objective lens 22 and reflected light from the substrate is detected by an image sensor 23. The detection is repeated while moving the stage at a constant speed, whereby an image is detected as a detected image data 24, which is then stored in memory 25. The detected image data 24 is then compared with stored image data 27 in the memory 25, which image can be expected to be the same pattern as the detected image. If the detected image is the same pattern, it is regarded as a normal portion, while if it is a different pattern, it is regarded as a pattern defect, represented by pattern defect data 11, and a defect position is determined.
As an example, FIG. 3 shows a layout in a case where the object substrate 5 is a wafer 31. On the wafer 31, there are formed dice 32 which are to be separated when the wafer is finally cut into individual products of the same type. The Z stage 6 is moved along a scanning line 33 to detect an image in a stripe area 34. If a detecting position A now lies at 35, an image 36 at a detecting position B in the memory 25 is taken out as stored image data 27 and is compared with a pattern which can be expected to be the same pattern. The memory 25 has a capacity capable of holding images which can be expected to have the same pattern. By using the memory 25 with a ring-like turn, an actual circuit is constituted.
In the following two examples inspection is performed in such a manner that, in synchronism with pattern detection for an object to be checked for a defect using a binary image, not only is it determined whether a pattern is defective, but also a defect in a specific mask area is ignored.
JP-A No. 278706/1986 discloses an example of inspecting through holes formed in a printed board. There is provided beforehand a printed board with a hole formed only in an area which is an area not to be inspected. An image of the printed board is detected before inspection and is made into a binary hole presence/absence image, thereby detecting whether masking is necessary or not, which image is stored as image data in a masking data storage unit. In the case where a place giving rise to a difference in binary image during inspection corresponds to the image area stored in the masking data storage unit, the difference is ignored and is not inspected thereby.
JP-A No. 5116/1995 discloses an example of inspecting a printed board. A pattern shape is detected and binarized. A normal/abnormal decision is made based on the detected pattern and a check is made to see if the detected pattern lies in a regular pattern. Only when a non-conforming pattern lies in the regular pattern is it judged to be abnormal.
In the following two examples, a dead zone is provided in a pattern boundary portion for the purpose of tolerating an error of the boundary portion in accordance with pattern information.
JP-A No. 146682/1990 discloses an example of inspection in which a mask pattern is compared with design information. A contracted image obtained by contracting a pattern by a certain width and an enlarged image obtained by enlarging the pattern by a certain width, in accordance with design information, are subjected to calculation, and a portion common to both is taken out, thereby providing a dead zone of a certain width. That is, inspection is conducted while setting a mask area so as to ignore an error of a certain width of the pattern boundary portion in accordance with design information.
JP-A No. 312318/1997 discloses an example of inspecting a pattern with use of a scanning electron microscope (hereinafter referred to simply as an “SEM”). In accordance with a reference image obtained beforehand, the vicinity of a pattern image is set as an area not giving rise to a defect because a minute displacement of a pattern edge is not a defect, and an image of an area not giving rise to a critical defect is not acquired. If a difference from the reference image is recognized in an image-acquired area, it is judged to be a defect.
JP-A No. 85742/1991 discloses an example of an apparatus which inspects a pattern on a printed board in a comparative manner. An image of a defect candidate obtained by a comparative inspection is stored in memory and a check is made to see if the defect candidate is a true defect or not asynchronously with inspection and on the basis of the stored image.
In JP-A No. 245161/1996, an object to be inspected, which has plural repetitive patterns, is compared with a candidate portion repeatedly, and when all the comparisons are found to be normal, it is judged that the result of inspection is normal. Comparison and decision are made with respect to N places of images at a time.
In JP-A No. 232250/1991, a simultaneous decision is made for both a cell comparison (repetitive pattern comparison) method for inspecting a portion having a repetition and a die comparison (chip comparison) method for inspecting the whole surface of each die.
For inspection, according to a broad classification, there are two types of requests, one of which is a stable inspection for establishing a statistical sense and the other of which is a high-sensitivity inspection applied in device development. In the former inspection for a statistical sense, for example, wafers in a specific process are checked periodically under the same conditions and the number of defects thereof is managed. This is generally a stable inspection method, which however is carried out in a low-sensitivity condition permitting detection of only relatively large defects. On the other hand, the inspection applied in device development is a high-sensitivity inspection performed for the purpose of detecting all defect modes and even minute defects.
For conducting these two inspections in the conventional inspection apparatus, it has been necessary to change inspecting conditions for the two types of inspections, thus requiring an increased inspection time for plural inspections. Alternatively, it is necessary to provide plural inspection recipes. In the case of a SEM type inspection apparatus, if inspection is conducted plural times, irradiation of the electron beam is carried out plural times, with the result that the state of the object changes due to electron beam irradiation and the inspection is inaccurate in such a changed state of the object. It is not believed that due consideration is given to those problems in the conventional inspection method and apparatus.
Further, the object to be inspected is fabricated in accordance with design information. The density, material and shape of a pattern are determined by design and deviate depending on place. If the pattern density differs, the amplitude in detected signal quantity of a detected pattern differs because there is a limit in the resolution of the inspection system. The same also occurs according to the pattern shape and material. Therefore, even in the case of defects of the same size, if inspection is performed with uniform sensitivity, there arise both a detected place and an undetected place because of different background patterns.
Moreover, even defects of the same size differ in point of whether they can be critical or not. That is, a defect of a place low in pattern density is less critical. Conversely, a place high in pattern density can be a critical defect even if it is a minute defect. Thus, the importance of a defect depends on the pattern density. There also occurs a difference according to material and shape. It is impossible to consider detection sensitivity, defect management and design information separately from one another. It is not considered that the conventional inspection apparatus and method give due consideration to these problems.
As to the optical type and electron beam type pattern inspecting apparatuses disclosed in the foregoing '247 and '703 publications, respectively, these disclosures merely indicate that the whole area is inspected using a single uniform condition for inspection.
According to the techniques disclosed in the foregoing '703 and '247 publications, an area not to be inspected involving a change in inspecting condition is established. In the example disclosed in the '706 publication, it is necessary that an area not to be inspected, which is included in a very large inspection area, should be set in terms of a bit pattern. This corresponds to 7T bits and thus requires a vast number of memories, assuming that an inspection area of 300 mm in diameter is inspected using 0.1 μm pixels in the case of application to wafer inspection. In the '116 publication, it is indicated that an area other than a regular pattern portion is set as an area not to be inspected. However, a wafer pattern for a semiconductor device is constituted by a very complicated pattern, so by utilizing a mere simple regularity, it is impossible to set an area not to be inspected, nor is it possible to obtain plural inspection results in a single inspection.
In the foregoing '682 and '318 publications, it is indicated that an area not to be inspected is set. But since this area not to be inspected, or a non-inspection area, is limited to pattern edges, it is impossible to set a non-inspection area at a required place, nor is it possible to obtain plural inspection results in a single inspection.
In the foregoing '742 publication, a method is described wherein image information of a defect candidate is preserved and an inspection is performed in detail on the basis of the preserved image information to determine whether the defect candidate is a true defect or not. This method can cope with a complicated pattern shape. However, a defect-or-not decision is made in accordance with one uniform criterion, and a portion which is not defective is considered to be a normal portion. That is, as to the portion which is once considered to be a normal portion, information is lost. Besides, plural inspection results cannot be obtained in a single inspection.
The methods described in the foregoing '161 and '250 publications are simultaneous inspection methods for N places, involving comparison with different places, in which results are not obtained under different inspection conditions for one and same place.